In a common type of machine tool, a cutting tool is connected to a rotatable drive shaft driven by the machine tool spindle. In many cases, it is desirable to supply coolant to the operating face of the cutting tool to help to cool it during use. Rotary coolant adaptors are used to do this and comprise two main parts; a rotatable shaft provided at one end with a chuck to receive the tool and at the other end with a connection to the machine tool spindle; and a fixed portion that locates around the rotatable shaft and connects to the main body of the machine tool and remains static when the shaft rotates.
The main function of rotary coolant adaptors is to transfer coolant from an inlet on the fixed portion of the adaptor to an inlet on the rotatable shaft of the adaptor, thereby allowing the coolant to pass through a bore in the tool and be ejected through apertures close to or in the operating face of the cutting tool.
As the machine tool spindle rotates at high speed, this can create a large amount of heat at the operating face of the cutting tool. It is therefore necessary to supply a sufficient volume of coolant to the operating face to keep the temperature of the cutting tool within working limits. As this entails using pressurised coolant, it is necessary to provide a seal in the rotary coolant adaptor to help to avoid leakage as coolant passes from the fixed portion of the adaptor to the rotatable shaft.
In the past, rubber lip seals having an edge which abuts the rotatable shaft have been used, these seals locating in housings provided in the fixed part of the adaptor and having edges for sealing against the rotatable shaft. However, these seals suffer from the disadvantage that they wear out quickly as they are pressed against the rotating shaft, which is typically made of metal. Further, the coolant used in such tools is recycled and, although it is often filtered to remove large fragments of waste material, inevitably small fragments remain in the coolant. These help to further abrade these rubber seals, further reducing their life.
In addition, the pressure of coolant that can be supplied through this type of rubber seal is limited, typically to a pressure of around 3.4 MPa (500 lbf/in.sup.2). Although in the past this has not been a problem, increasingly modern tools are required to operate at higher rates and/or more intensively, requiring increased flow of coolant to provide the desired cooling effect at the operating face of the cutting tool.
There have been many attempts to increase the pressure of coolant ejected from the rotatable shaft near the tool. However, these attempts have used the known seals and have tried to increase the coolant pressure as it passes through the bore in the shaft. Thus, U.S. Pat. No. 4,652,189 (Mitzoguchi) discloses a conventionally sealed device which relies on a rotor in a chamber in the shaft to increase the feed pressure of the coolant that has already passed into the shaft through the adaptor. Another arrangement is disclosed in U.S. Pat. No. 4,598,617 (Kubo et al) which shows an adaptor having an eccentric rotor in the bore of the rotatable shaft which is driven by the machine tool spindle to create a pumping action to help keep the pressure to the coolant supplied through the shaft constant.
Further, although attempts have been made to replace the rubber seals with seals made from materials having increased frictional wear resistance, eg graphite, none of these attempts has provided a seal having both good wear resistant properties and which is capable of sealing higher pressure systems successfully during long periods of use. There is also an increasing requirement for machine tools to operate under more extreme conditions that require higher coolant flow rates and pressures.